1. Field of the Invention
The present invention relates to a method for making titanium alloy products having high strength and ductility.
2. Description of the Related Arts
Titanium alloys have been widely used in aerospace applications for their advantages of high ductility and strength. In recent years, they have also been introduced in consumer product applications. High-strength type titanium alloys, typical of which is Ti-6Al-4V, however, have a disadvantage of high working cost due to their poor workability in general.
To overcome such a disadvantage, a superplastic forming/diffusion bonding method has been developed and used as a new forming method ("A Study on Fabrication Method of Integrated Light Titanium Sheet Metal Structure by Superplastic Forming/Diffusion Bonding", Makoto Ohsumi et al., Mitsubishi Heavy Industries Technical Review Vol. 20, No. 4, (1983-7), hereinafter called Prior Art 1). This forming method is to heat a titanium alloy to a predetermined temperature in .alpha.+.beta.-phase, and to form it at a low strain rate, by which a component of a Final product shape or its similar shape can be formed.
However, the above-described forming method has problems as described below. For the most widely used Ti-6Al-4V alloy, the structure becomes coarse due to grain growth during superplastic forming because the superplastic forming temperature is as high as a temperature from 900.degree. to 950.degree. C., so that deterioration in mechanical properties (for example, decrease in strength and ductility) occurs.
For the Ti-6Al-4V alloy, the strength can be increased by rendering heat treatment of solution treatment and aging, but rapid cooling such as water quenching is needed in cooling after solution treatment. Therefore, it is almost impossible to apply this alloy to superplastically formed components. The superplastic forming is mainy applied to thin sheets. If a sheet component undergoes water quenching, quenching strains due to thermal stresses are developed, so that the component cannot function as a product.
Further, for the Ti-6Al-4V alloy, the reduction in forming cost is limited because of its high forming temperature. Therefore, the development of a titanium alloy which allows superplastic forming at lower temperatures has been attempted ("Enhanced Superplasticity and Strength in Modified Ti-6Al-4V Alloys", J. A. Wert and N. E. Paton, Metallurgical Transactions A, Volume 14A, December 1983, p.2535-2544, hereinafter called Prior Art 2).
In accordance with the requirements shown in Prior Art 2, some of the inventors of the present invention have developed a titanium alloy for superplastic forming which has a superplastic forming temperature 100.degree. C. or more lower than that of the above-described Ti-6Al-4V alloy (Japanese Unexamined Patent Publication Laid-Open No. 3-274238, hereinafter called Prior Art 3). Specifically, the use of an alloy, whose typical composition is Ti-4.5Al-3V-2Mo-2Fe, remarkedly decreases the superplastic forming temperature.
In the above-mentioned Prior Arts 1 to 3, however, the following four problems remain to be solved.
Firstly, quenching strains are developed in solution treatment after superplastic forming, and high strength and ductility cannot necessarily be obtained by solution treatment and subsequent heat treatment.
Secondly, in terms of cost, it is undesirable to repeat the solution treatment on a superplastic component. Therefore, the establishment of an alternative, efficient manufacturing technique is expected.
Thirdly, deterioration in material properties takes place due to superplastic forming, so that their strength and ductility are prone to decrease.
Fourthly, the establishment of a superplastic forming/diffusion bonding process is expected so that it can achieve excellent diffusion bonding strength.